The pion: an enigma within the Standard Model

摘要: Quantum Chromodynamics (QCD) is the strongly interacting part of Standard Model. It supposed to describe all nuclear physics; and yet, almost fifty years after discovery gluons quarks, we are only just beginning understand how QCD builds basic bricks for nuclei: neutrons protons, pions that bind them together. characterised by two emergent phenomena: confinement dynamical chiral symmetry breaking (DCSB). They have far-reaching consequences, expressed with great force in character pion; pion properties, turn, suggest DCSB intimately connected. Indeed, since both a Nambu- Goldstone boson quark-antiquark bound-state, it holds unique position Nature and, consequently, developing an understanding its properties critical revealing some very features We experimental progress toward meeting this challenge has been made using electromagnetic probes, highlighting dramatic improvements precision charged-pion form factor data achieved past decade new results on neutral-pion transition factor, which existing notions structure. also provide theoretical context these empirical advances, begins explanation works guarantee unnaturally light; but also, nevertheless, ensures best object study order reveal mechanisms generate nearly mass hadrons. In canvassing advances areas, our discussion unifies many aspects structure interactions, connecting elastic pion's leading-twist parton distribution amplitude. sketches novel ways studies charged-kaon can significant contributions. Importantly, appears recent predictions large-Q 2 behaviour charged- be tested experiments planned at upgraded 12GeV Jefferson Laboratory. Those will extend precise up momentum transfers now may large enough serve validating factorisation theorems QCD. If so, they expose between nonperturbative perturbative domains thereby reach goal driven hadro-particle physics around thirty-five years.